1. Field of the Invention
This invention relates to rain gauges, and in particular to a precise rain gauge and methods of calibration.
2. Background of the Invention
Rain gauges are instruments used to measure the amount of rain that falls in a certain place or region during a given period of time. This measurement function is important, because the resultant data may be used to determine the amount of extra irrigation required by crops, to forecast rainfall trends to help plan what specific types of crops to plant, as well as to aid in designing storm-water handling systems and flood control, etc.
3. Existing Designs
A simple rain gauge may comprise a funnel whose output flows into a graduated cylinder with markings calibrated to yield a reading of how many inches of rain have fallen. A more sophisticated design which provides greater accuracy is the tipping buckets design. A tipping bucket rain gauge incorporates a pair of tipping buckets adjacent to each other. The buckets are separated by a dam. In operation, the output of a funnel fills the first bucket. When the first bucket is filled to a pre-determined amount of water--typically 8.238 milliliters (ml) which is equivalent to 0.01 inch (in) of rain--the bucket pair tips, and the second bucket commences to receive the funnel output. The tip is sensed by a hull switch, and recorded by a counter. Total inches rainfall over a period of time equals the number of tips multiplied by 0.01 in.
It is important that the time required to tip the first bucket be as close as possible to the time required to tip the second bucket, given the same rainfall rate. Any variation between tip times of the two buckets contributes to gauge inaccuracy, because water will fall into one bucket longer than into the other. This contributes to a small error with each tip, which can cause an appreciable cumulative error over time. Within the current state of the art, presently available rain gauges are capable of tip time calibration which brings the tip times of the buckets to with 3 seconds (secs) of each other. This time difference contributes to an appreciable cumulative rainfall measurement error over time.
Another problem associated with current tipping bucket rain gauge designs is the inability to accurately calibrate them for different rain intensities. Most commercially available designs are delivered with a set flow rate calibration, typically in the 2-6 inches/hour (in/hr) area. The only method disclosed by the manufacturers to change this calibration is to replace the buckets with different sized buckets.
If the rain were to increase in intensity, for example to 12 in/hr, the momentum of the heavy rainfall imparted to the bucket into which the rainfall were falling would cause pre-mature tipping, resulting under-measurement of the actual rainfall. The instructions for use of currently available rain gauges requires replacement of the buckets with larger ones to accommodate heavier rainfalls flow rates. No method of calibration is currently taught which permits flow rate calibration of a rain gauge without replacing the buckets.
A further problem associated with presently available rain gauges is the lack of a firm funnel mount above the buckets. This deficiency renders funnel replacement cumbersome and difficult, because the rain gauge must be re-calibrated with every funnel replacement, because the replacement funnel output is frequently in a deferent position compared to its predecessor.
Finally, currently available rain gauges incorporate opaque covers which render the inner workings invisible. This visual obstruction makes visual monitoring of the rain gauge operation impossible without removing the entire cover.